This invention relates to a process for the continuous deposit-free polymerisation of chloroprene in aqueous emulsion in the presence of small quantities of emulsifiers to form stable latices and solid rubbers having improved raw material, mixing and vulcanisate properties.
The polymerisation of chloroprene in aqueous emulsion has long been known and is carried out on an industrial scale (cf. for example Encyclopedia of Polymer Science and Technology, Vol. 3, pages 705-730, Interscience, 1965). This radically controlled polymerisation process takes place very quickly and is accompanied by vigorous evolution of heat. Accordingly, rapid dissipation of the heat of polymerisation is difficult and presupposes a very efficient cooling system. Numerous proposals have been put forward in the patent literature with a view to meeting the stringent demands which are imposed upon the polymer. However, difficulties are involved in the reproducible production of polychloroprene having a uniformly high quality level. Accordingly, it is preferred in practice both for this reason and for economic reasons to polymerise chloroprene in a continuous polymerisation plant.
As those skilled in the art are aware, the raw material and vulcanisate properties of the polymers depend upon the composition of the emulsifier and the concentration thereof in the aqueous emulsion. Although a high emulsifier content increases the polymerisation velocity and improves the stability of the latex, it has an adverse effect upon crucial product properties and, for example, leaves the crude rubber having considerable tackiness on mixing rolls. Accordingly, it would be desirable to reduce the emulsifier content of the polymerisation mixture. As is known, however, the polymerisation velocity decreases with reduced emulsifier content (F. Holscher Dispersionen synthetischer Hochpolymerer [Dispersions of Synthetic High Polymers], Part I, Properties, Production and Testing, Springer-Verlag, Berlin-Heidelberg-New York, 1969, pages 81 et seq). Where polymerisation is carried out continuously, this means a lower monomer conversion in the polymerisation plant for the same residence time.
The monomer conversion may be increased for the same polymerisation temperature either by a longer residence time of the emulsion in the polymerisation plant or by using larger amounts of activator. In either case, reductions in product quality are inevitable.
The mean residence time (V.sub.z) of the polymerising chloroprene in the reactor cascade is crucially important if polymerisation is to take place uniformly without interruption and if the polymer is to have uniformly good properties. It is determined in accordance with the following simple formula: EQU V.sub.z =(.SIGMA.V.sub.k /L)
wherein
V.sub.z is the residence time [h]; PA1 .SIGMA.V.sub.k is the sum of the reactor capacities of the polymerisation plant [m.sup.3 ]; and PA1 L is the quantity of latex formed per hour [m.sub.3 /h].
In a cascade of 7 reactors, V.sub.z is normally from 2 to 4 hours. Where the emulsion has a long residence time in the continuous installation (V.sub.z &gt;5 hours), i.e. where the polymerisation reaction takes place slowly, so-called "popcorn" polymers are formed in addition to the required chloroprene polymer. Popcorn polymers consist of highly cross-linked products which are insoluble in organic solvents. Once popcorn seeds have formed, they grow quickly under the effect of autocatalysis, resulting in blockages in the polymerisation plant, excessive conversion levels and, hence, in the formation of polymers characterised by highly fluctuating properties.
If, by contrast, the residence time is too short (V.sub.z &lt;2 hours), the heat of polymerisation is difficult to dissipate and products having unfavourable raw material and vulcanisate properties are formed. Accordingly, the continuous interference-free polymerisation of chloroprene to form products of high quality is only possible within a certain residence time range.
This continuous polymerisation of chloroprene is known and has been described, for example in U.S. Pat. Nos. 2,384,277; 2,394,291 and 2,831,842.
In order to reach the same reaction velocity in continuous polymerisation as in batch polymerisation, it is either necessary to use more emulsifier or, as described in U.S. Pat. No. 2,394,291, to introduce considerably more activator solution. However, this leads to the formation of deposits in the polymerisation and degassing tract of the polymerisation plant.
Accordingly, continuous interference-free polymerisation in the presence of low concentrations of emulsifier is not possible.
Neither does the continuous polymerisation of chloroprene by means of an emulsifier system of salts of disproportionated abietic acid, fatty acid and non-ionic emulsifiers, as described in DE-OS No. 2,520,339, produce the required result because the non-ionic emulsifiers slow down the polymerisation reaction and the above-mentioned adverse effects occur.
German Offenlegungsschrift Nos. 2,047,449 and 2,047,450 describe processes for the production of polychloroprene latices rich in solids. In these processes, adequate colloidal stability of the latex during polymerisation is only achieved when a combination of 3 emulsifiers and dispersants in precisely defined concentration ranges is used. To obtain adequate latex stability, it is essential that, in addition to the polymeric fatty acids, no less than from 1.5 to 2.0 parts, by weight, of a condensation product of naphthalene sulphonic acid and formaldehyde per 100 parts, by weight, of chloroprene and no more than 1.5 parts, by weight, of a dispersed abietic acid should be added. However, these latices cannot be worked-up into the solid rubber, for example, by low temperature coagulation.
However, products obtained by electrolyte precipitation undergo serious discolouration during drying. Accordingly, it was surprising to find that it is possible, without adding polymeric fatty acids and in the presence of small quantities of condensation products of naphthalene sulphonic acids and formaldehyde, to improve latex stability during the continuous polymerisation reaction, considerably to lengthen the operating time of the polymerisation plant (even where the mixture has a low water content) and to obtain latices which may be readily worked-up, for example, by low temperature coagulation, providing the sodium hydroxide normally used is replaced by potassium hydroxide or by mixtures of other alkali metal hydroxides, such as LiOH, KOH, and CsOH, and the Na-salts of the emulsifiers are replaced by the potassium salts thereof. The reaction velocity of the continuous polymerisation of chloroprene in the presence of low concentrations of emulsifier may be adequately controlled by the choice of the mixture of different alkali metal hydroxides and the concentration thereof in the mixture. Storable latices are obtained in this way and, after working-up into the solid rubber, give products having considerably improved raw material and vulcanisate properties, such as low tackiness on mixing rolls.